Porous polytetrafluoroethylene polymer materials that have electrically conductive metals plated on them are well known. (See, for example Manniso U.S. Pat. No. 4,557,957.) They find use in the electrochemical field as electrodes in solid electrolyte, gas diffusion electrodes, etc. In addition, they are useful as electromagnetic shielding and as filters or circuit boards. But since polytetrafluoroethylene is highly water repellent and has extremely strong non-stick characteristics, its metallization is very difficult with common metal plating technologies, and even if this plating can be accomplished, the metal cover film is sometimes inadequate, irregular, and unstable. The metallization of the interior surfaces of a porous structure is particularly difficult, and this is even more pronounced when the size of the openings in the porous structure is less than 10 um.
As a means of solving these problems, the prior art has proposed a procedure in which porous polytetrafluoroethylene is immersed in a water-soluble surfactant solution, then the solution replaced with a metal activator means of liquid-liquid substitution, so that a non-electrolytic plating solution can enter the pores of the porous material, and both the surface of the material and the interior of the porous structure are plated.
Platinum metals and platinum group alloys exhibit favorable characteristics as electrode materials, that are used in the electrochemical industry, and also perform well when used in sensors, electromagnetic shielding materials, catalyst materials, printed substrates, antennas and other transmitter-receiver materials, and the like.
Therefore, although the above-mentioned plating technique is indeed useful, said technique requires that the porous polytetrafluoroethylene material be hydrophilicized with a watersoluble activator, and since this water-soluble activator will elute into an aqueous solution, the material loses much of its hydrophilicity just by being immersed in and then taken out of an aqueous solution. Consequently, the plating must be performed by carrying out a liquid-liquid substitution after the hydrophilization. This requires that the time, degree of immersion, pressure, and other factors be controlled, and since a non-electrolytic plating process generally involves repeated immersions into an aqueous solution, the substitution has to be repeated each time, thus requiring a number of steps. Furthermore, the control of these factors is not an easy task.
Also, to perform the plating, the object to be plated is, for example, immersed in a strongly acidic aqueous solution of tin(II) chloride to cause Sn.sup.2+ to be adsorbed on the surface, after which the object is rinsed with water and then immersed in a strongly acidic palladium chloride aqueous solution. A small amount of palladium is deposited on the surface of the object, and the object activated. The drawback in this case, however, is that it is difficult to deposit enough palladium to achieve activation with just one immersion, and in actual practice this process has to be repeated. The activation tends to be particularly uneven when activating a film substrate.
Moreover, the only materials that will form a suitable plating on a non-metal with this method are cobalt, copper, and nickel.
Meanwhile, with platinum metals and platinum group alloys, since the reduction of platinum group ions proceeds more rapidly than with other metals, the stability of the plating bath is poor and it is difficult to cause platinum to be selectively deposited on only the object to be plated. For these and other reasons, plating is extremely difficult with platinum metals and platinum group alloys.
Further, plating of the interior surfaces in a porous structure is difficult, and even if such plating can be accomplished, the deposition is sometimes not satisfactory.